Thread: Over zinc'd?
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Old 02-24-2018, 05:59 PM   #6
firstbase's Avatar
City: Jupiter, Florida
Country: United States
Vessel Name: Black Eyed Susan
Vessel Model: Grand Banks 42' Classic
Join Date: Nov 2016
Posts: 1,402
Originally Posted by Larry M View Post
I posted this on your other thread. For a fiberglass boat with inboards you should be reading -750mv to -1000mv. Less than that you are under protected. Are you in brackish water? If you are, you can change from zincs to aluminum anodes and that should get you a little more negative.

Edit: Edit: I checked ABYC recommendations. They say you are ok at -550 my. There are a lot of references out there.

ABYC Recommended range of cathodic
protection for boats of different hull
materials in saltwater
Hull Material Millivolt Range
Fiberglass -550 to -1100
Wood -550 to -600
Aluminum -950 to -1100
Steel -850 to -1100

Some info from
Corrosion Reference Electrode: Corrosion Quiz
Thanks Larry. I am in salt water, about a mile from the Atlantic and 200 yards from the ICW so nothing brackish about it. When all is said and done I am going to get him to show me the "after" tests as well as ask about the 30ma dock breaker, how do I know that I won't set it off. I assume that would be due to current leaving my boat and going to the dock pedestal and tied to the ground/neutral situation? Something allowing current to go out the back door.
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